The present invention relates to the control of photodetector signals, particularly signals from photodetectors which are used as sensors in receiving radiation passing through some portion of an imaging system. More particularly, the signals controlled in the present invention are specially connected with sensors for sampling electromagnetic radiation transmitted through the system optics.
The use of charge-transfer device technology to process signals obtained from photodetectors has a number of advantages. First, charge-transfer devices, particularly charge-coupled devices, can be relatively easily fabricated in silicon monolithic integrated circuits, and can be fabricated such that these devices individually are capable of being provided therein in a high density. The charge-transfer device with respect to analog signals, is basically a sampling device directly manipulating the analog samples. Thus, interface between such a device and the photodetector can be relatively uncomplicated since the photodetector, typically, provides an analog electrical output signal more or less related to that electromagnetic energy which has been sensed therein. Further, the analog samples to be manipulated in the charge-transfer device can be controlled by digital clocking circuits which permit considerable flexibility in treating the analog samples.
If charge-coupled device technology is to be used for processing the signals resulting from the photodetector, a very convenient photodetector to use is a conductor-insulator-semiconductor (CIS) detector which is essentially a capacitor. In such photodetectors, the semiconductor material supports an insulator which in turn supports the conductor, the side of the conductor opposite its insulator support being first exposed to impinging electromagnetic radiation of interest for detection. The conductor is an electrode which together with the insulator are of such a nature as to permit radiation to pass therethrough to reach the surface of the semiconductor material. With the voltage applied between the conductor serving as a photodetector electrode and the semiconductor material such as to form a depletion region in the semiconductor material, charges accumulate at the surface of the semiconductor material at the sensing site in proportion to the amount of radiation experienced at that semiconductor material surface. This radiation induced charge is accumulated and held at the semiconductor material surface at the sensing site for the time duration that voltage is maintained on the conductor because of the potential resulting at the semiconductor material surface due to this applied voltage. In a typical photodetector system, the conductor voltage is typically a repeated voltage pulse changing between zero voltage and some applied voltage level, as just indicated, with such a voltage pulse being provided to the conductor at each sensing site. Thereby, sampling is accomplished of the incoming radiation at various points across a phase surface thereof in the sampled data photodetector system.
However, there is a maximum amount of radiation induced charge accumulation that is desired at a sensing site, either because (i) the photodetector cannot accumulate any further charge at the sensing site for the voltage present there, or because, (ii) the charge-coupled device signal processing circuitry design can operate only with a certain maximum amount of accumulated charge in a period of time, i.e. a maximum size charge packet representing a sample in a sampling period. One possibility, then, would be to provide a fixed time duration for accumulating charge induced by radiation impinging at a sensing site in a sampling period. Such a procedure cannot always be relied on to prevent an overaccumulation of charge during a sampling period at a sensing site. This is because the intensity of the radiation impinging on the sensing site will often be unknown because the scene being imaged will usually have a substantial variety of radiation intensities thereacross which in many instances cannot be predicted, either as to the absolute intensity maximum that will occur or as to the locations of intensity maxima in the scene. Thus, the maximum amount of charge which will accumulate in a fixed time duration in a sample cannot always be predicted either, nor can the particular sensing site be predicted at which such a maximum sample will occur.
Another possibility would be to transfer the accumulated charge, or sample, from each sensing site into a sequential position in a charge-coupled device shift register and monitor the size of each charge packet as is transferred by a selected monitoring point. Then, the time duration could be varied depending on the size of the charge packets coming by the monitoring point. However, the result is that the charge packets do not have a maximum size selected for them during the sampling period the packet is being accumulated raising the possibility that too large a charge accumulation will occur during that sampling period before the monitor senses the situation. Further, the monitoring process takes additional time which can interfere with the photodetector signal processing insofar as limiting the rate of changes which can be sensed in the scene being imaged because of the limited time response of the photodetector signal processing system.
Thus, a photodetector signal system would be desirable in which the size of the charge packets occurring at the various sensing sites is controlled at the very time during which these charges are being accumulated. Further desired would be a photodetector signal processing system in which the charges accumulating in the packets of each sensing site would be simultaneously monitored so that the size of the charge packets at every sensing site in the array would affect the decision as to when sensing should be terminated during any particular sampling period, i.e. when the sensing portion should terminate in a particular sample period or frame.